Renaissance Periodization Blog

by Nick Shaw on Jun 24 2015

In-Depth On Fiber – Tiago Vasconcelos

Fiber Terminology

Fiber is a type of carbohydrate. It’s generally not considered a macro-nutrient but is a sub-type of carbohydrate, and it’s not needed for survival. The health effects of fiber are quite varied and are determined to a large extent by the types and amounts present. There are several ways to classify fibers, the most common being soluble and insoluble. Fibers that dissolve in hot water are soluble, and those that do not dissolve in hot water are insoluble. Soluble fibers can bind nutrients and water, cause gel formation, and can be fermented, while insoluble fibers can only bind water and be degraded.

In some textbooks you can note that fiber being referred to as fermentable vs non-fermentable. Fiber reaches the colon undigested by human digestive enzymes. While both soluble and insoluble fibers can be fermented, soluble fibers are usually fermented by colonic microflora to a greater degree than insoluble fibers. Some fermentable fibers act as prebiotics (promoting the colonic growth and health-promoting bacteria) and generate short-chain fatty acids for use by the body, which also has benefits such as increased water and sodium absorption through the colon. Non-fermentable fibers also have their share of health benefits, mainly for colonic health. They move through the gastrointestinal tract unchanged and are excreted. They play an important role in detoxification and they increase the weight and size of your stool, softening it and normalizing bowel movements, which can be an important implication as around 13% of the U.S. population is semi-chronically constipated [1]. A bulky stool is easier to pass, decreasing your chance of constipation. If you have loose, watery stools, fiber may also help to solidify the stool because it absorbs water and adds bulk to stool. However, there’s also evidence that stopping or lowering the intake of dietary fiber can effectively reduce idiopathic constipation, especially if the individual fiber intake is already high [2]. Whether or not lowering or raising fiber enhances bowel movements is likely to be greatly individually-based, so playing around with fiber intake by slowly increasing or decreasing it might be a good start.

In the 2002, the National Academy of Sciences Food and Nutrition Board wrote an article titled “Dietary Reference intakes for Energy, Carbohydrate, Fiber, Fat, Protein, and Amino Acids.” Within that article the term “functional fiber” was introduced, which is not exactly synonymous with dietary fiber. Dietary fiber refers to non-digestible carbohydrates from digestive enzymes and lignin that are intact and intrinsic in plants. Functional fiber consists of non-digestible carbohydrates that have been isolated, extracted, or manufactured and have been shown to have beneficial physiological effects [3]. This includes natural mucilages such as psyllium; chitin from the exoskeletons of crustaceans; synthetic fructooligosacchrides, polydextrose, and polyols; and resistant dextrins. The term fiber that appears on food labels reflects only dietary fiber, not functional fiber per se.

Finally, there’s the classification of viscous and non-viscous fiber. Viscous fibers are the ones that form viscous solutions or gels in water. They slow the emptying of the stomach, delay the absorption of some nutrients in the small intestine, and seem to lower serum cholesterol [4]. Almost all viscous fibers are soluble, with the notable exceptions of fructans and some hemicelluloses. All non-viscous fibers are insoluble, and therefore, they share the same benefits. Viscous fibers include pectins, β-glucans, some gums and mucilages, while cellulose, lignin, and some hemicelluloses are nonviscous fibers.

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Fiber Dietary Benefits

Probably the greatest benefit of a high fiber diet is the feeling of fullness/satiation that comes with higher fiber intakes. This satiation effect typically allows an individual to feel full while eating fewer calories. This occurs from the stretching of the stomach when consuming a high fiber meal, coupled with the slowed gastric emptying from soluble fiber, the latter accomplished by the fiber attracting water and forming a viscous gel during digestion. [5]

Diets rich in fiber are beneficial to varying degrees in the prevention and management of several health problems, and also balance intestinal pH and reduce risk of colorectal cancer by stimulating intestinal fermentation production of short-chain fatty acids [6].

High fiber diets have been linked to a lower risk of heart disease [7], coronary heart disease [8, 9] and metabolic syndrome [10, 11]. They have also been shown to decrease the risk of developing Type 2 Diabetes [12-15] and diverticulitis [16], which is an inflammation of the intestine, one of the most common age-related disorders of the colon in Western society.

Some clinical trials that have found that increasing the intake of viscous dietary fibers decreases serum total and LDL cholesterol [17-25]. A meta-analysis that combined the results of 67 controlled trials found that even a 10-g per day increase in viscous fiber intake resulted in reductions in LDL-cholesterol averaging 22 mg/dL (0.57 mmol/L) and reductions in total cholesterol averaging 17 mg/dL (0.45 mmol/L) [26]. There’s also some evidence suggesting it can reduce the incidence of several types of cancer, but the evidence goes both ways [27-61], more research is needed to determine the complex effects of dietary fiber and fiber supplements on cancer. Some studies have also found higher intakes of dietary fiber to be associated with a lower risk of mortality from all causes [62-66], but in a follow-up study in 2003, no association was found between total fiber intake or soluble fiber intake and mortality [67], so the balance of the evidence on fiber and all-cause mortality is at this point still unclear.

The evidence for weight control and altering body composition is inconclusive [68-76]. It’s unlikely that simply introducing fiber to one’s diet can meaningfully alter body composition. However, it certainly makes dieting easier by increasing fullness and therefore indirectly reducing overall caloric intake. Since consistency is likely the biggest factor of determining dieting success, having an adequate fiber intake is key in accomplishing body composition goals.

Energy Value of Fiber

There’s a common belief that fiber has 4 calories per g since it’s a carbohydrate, or that fiber does not get digested, therefore not containing any calories. Both extremes are false. It is true that fiber cannot be fully absorbed by the body, since we lack enzymes to break it down. However, the degree to which our bodies can break down the fiber depends on the fiber type and is not “zero” for all fiber types. Insoluble fiber does not change inside the body, the body cannot absorb it and it provides no energy. Soluble fiber is partially fermented, with the degree of fermentability varying with the type of fiber, and contributes some energy when broken down and absorbed by the body.
There’s no consensus on how much energy is actually absorbed, but experts tend to agree it’s likely between 1.5 to 2 calories per gram of most soluble fibers. In some countries all fiber must be listed and is considered to provide 4 calories per gram just like any carbohydrate, while in other countries (such as the US) soluble fiber must be counted as 4 calories per gram, but insoluble fiber might be considered as calorie-free and not mentioned on the label. That means that if you live in the U.S., you can continue to count insoluble (listed on the nutrition label) fiber as zero calories and be pretty close to correct.

Perhaps the belief that fiber doesn’t contain any calories at all started because certain fibers may impair nutrient absorption by reducing the absorption of carbs, fats, and proteins. This was hypothesized to make your overall daily caloric intake lower since not all of the eaten calories are absorbed. So assuming that the reduced absorption will lead to a greater decrease in daily caloric intake than the calories consumed from fiber itself, it’s possible that fiber would actually make you lose weight, or if the calories are the same, it would make no difference. However, this is highly unlikely, so it might not be a good idea to count on. For nutrients to be absorbed they must move from the lumen of the small intestine through water layer lying on top of the enterocytes and finally into the enterocyte (a type of cell that absorbs water and nutrients from the digestive tract). The increased thickness of the unstirred water layer due to the presence of the viscous fiber solution decreases the diffusion rate of nutrients through this water layer. There also might be other mechanics responsible such as delayed glucose absorption, but so far as we know, the only slow the absorption of glucose and other nutrients, they don’t stop it or reduce it.

The total effect of fiber interfering with macronutrient absorption is rather small. However, it’s a different case for micro-nutrients, especially electrolytes (K, Na, Mg and Ca). This isn’t a huge deal if your fiber intake isn’t excessive and/or if the majority of your fiber comes your whole food sources, such as vegetables, fruits, nuts and so forth. Most of these foods will contain electrolytes to begin with. But problems may arise when people start trying to “artificially” bump their fiber intake. Lately this is happening quite a lot in some fitness circles. As the IIFYM craze took over, more people started to diet down with more “dirty” foods. And if you start to consume more “dirty” or processed foods, you will have to consume less whole or “good” foods by default, thus lowering your fiber intake.

Fiber and Micronutrient Intake

However, flexible dieting and IIFYM philosophy places a large emphasis on fiber intake, so people started to increase their intake from more purified sources, such as fiber bars or high fiber breakfast cereals to offset this. This might not be the best idea because they are decreasing mineral absorption by having more fiber, but not compensating by consuming more nutrients, since that extra fiber is being consumed from sources that aren’t very nutrient dense. This is why most of the time I don’t recommend quest bars or any other cookie cutter fiber foods for my clients. Don’t get me wrong, I’m not saying you can’t eat these foods, or that increasing your fiber intake from it is bad, but it should be done in moderation, if you’re getting 40g of fiber a day and 30g are coming from quest bars, that’s obviously not moderation. Think of it as whey protein powder. If you get a scoop or two scoops to bump your protein intake, that’s reasonable, but if more than half your protein intake comes from shakes, then you just need to start eating more protein-rich foods.

It’s worth mentioning that there is hypothesis that fiber does not restrict nutrient absorption, as it does not bind to minerals and vitamins [77-79]. It states that the reduced absorption is caused not by fiber, but by phytate [80]. However most fiber sources include phytate anyway, so even if it’s the culprit, it wouldn’t change much. Phytate has its own health benefits such as prevention of cancer, cardiovascular disease and insulin resistance.

Dietary Fiber Recommendations

There’s no globally accepted optimal intake for fiber, but we do have some recommendations from reputable organizations. The Institute of Medicine recommends 38g for men and 25g for women, while the American Dietetic Association recommends an intake of 20 to 35 g, and the American Heart Association recommends 25 to 30 g daily. Sometimes the recommended intake also varies by age (both children and the elderly consume less fiber) or total calories consumed (the higher the calories, the higher the fiber intake). In general, despite everyone recommending slightly different values, it points to somewhere around 20 to 40g. It’s likely that you can consume more than 40g without any issues, as long as it comes mainly from whole food sources, and that the intake isn’t excessive. I’d consider 60-80g the likely limit, assuming you have a medium to high caloric and carb intake (it’s probably difficult do without it anyway), you don’t feel any negative effects, and you have built up to such a high intake overtime. Since fiber absorbs water, beverage intake should be increased as fiber increases as well. As usual and typical of the fitness community, people obsess with a very specific fiber value but that’s focusing on an irrelevant detail, just make sure you end up in the ranges mentioned and you’re good to go.

The average adult only eats 12–18 grams of fiber per day [81], which is lower than desired. If your intake is much lower than recommended, it could be a bad idea to immediately increase it. Abdominal discomfort, bloating, gas, and altered stool output are commonly reported if you increase it too fast. Slowly get your body accustomed to a higher fiber intake. For example, if you are currently eating around 10g and you plan to start hitting 40, focus on hitting 15-20 first, and slowly build your way up.

A lot of people have troubles increasing their fiber intake once they realize it’s too low. More vegetables and legumes is obviously the main and preferred route, but in my experience people often overlook fruit. It has a decent amount of fiber with not too many calories, and it’s normally easy for people to consume a greater amount of it since fruit is naturally sweet and enjoyable, as opposed to veggies, which in many cases are only consumed because they have to be. Whole grains are also a good option, and nuts have some fiber too if you can afford the extra calories. Another option is fiber bars or fiber-rich cereals, but like as previously mentioned, that option has its own set of problems, so try to avoid it and keep it in moderation. In order to have all the benefits of every type of fiber, it’s recommended that your fiber intake be composed of several sources.